Reflective polarizing films transmit light of one polarization and reflect light of the orthogonal polarization. They are useful in an LCD to enhance light efficiency. A variety of films have been disclosed to achieve the function of the reflective polarizing films, among which diffusely reflecting polarizers are more attractive because they may not need a diffuser in a LCD, thus reducing the complexity of the LCD. U.S. Pat. Nos. 5,783,120 and 5,825,543 teach a diffusely-reflective polarizing film comprising a film containing an immiscible blend having a first continuous phase (also referred herein as the major phase, i.e., comprising more than 50 weight % of the blend) and a second disperse phase (also referred herein as the minor phase, i.e., comprising less than 50 weight % of the blend), wherein the first phase has a birefringence of at least 0.05. The film is oriented, typically by stretching, in one or more directions. The size and shape of the disperse phase particles, the volume fraction of the disperse phase, the film thickness, and the amount of orientation are chosen to attain a desired degree of diffuse reflection and total transmission of electromagnetic radiation of a desired wavelength in the resulting film. Among 124 examples shown in Table 1 through Table 4, most of which include polyethylene naphthalate (PEN) as a major and birefringent phase, with polymethyl methacrylate (PMMA) (Example 1) or syndiotactic polystyrene (sPS) (other examples) as a minor phase, except example numbers 6, 8, 10, 42-49, wherein PEN is a minor phase and sPS is a major phase. In all of these 124 examples the major phase comprises a semicrystalline polymer.
Examples 6, 8, and 10 in Table 1 showed that overall transmittance and reflectivity were not satisfactory. A figure of merit (FOM) defined as FOM=Tperp/(1−0.5*(Rperp+Rpara)) was smaller than 1.27. Examples 42-49 in Table 2 did not have the transmittance and reflectivity data, and were not discussed at all.
Fraction sPSTperpT_paraR_PerpR_paraFOM60.7580.258.419.4401.1480.75764123.855.61.26100.7576.848.922.449.61.20(Table 1 of U.S. Pat. Nos. 5,783,120 and 5,825,543)
U.S. Pat. Nos. 5,783,120 and 5,825,543 also summarize a number of alternative films that are described in the prior art, these are discussed below.
Films filled with inorganic inclusions with different characteristics can provide optical transmission and reflective properties. However, optical films made from polymers filled with inorganic inclusions suffer from a variety of infirmities. Typically, adhesion between the inorganic particles and the polymer matrix is poor. Consequently, the optical properties of the film decline when stress or strain is applied across the matrix, both because the bond between the matrix and the inclusions is compromised, and because the rigid inorganic inclusions may be fractured. Furthermore, alignment of inorganic inclusions requires process steps and considerations that complicate manufacturing.
Other films, such as that disclosed in U.S. Pat. No. 4,688,900 (Doane et. al.), consists of a clear light-transmitting continuous polymer matrix, with droplets of light modulating liquid crystals dispersed within. Stretching of the material reportedly results in a distortion of the liquid crystal droplet from a spherical to an ellipsoidal shape, with the long axis of the ellipsoid parallel to the direction of stretch. U.S. Pat. No. 5,301,041 (Konuma et al.) make a similar disclosure, but achieve the distortion of the liquid crystal droplet through the application of pressure. A. Aphonin, “Optical Properties of Stretched Polymer Dispersed Liquid Crystal Films: Angle-Dependent Polarized Light Scattering, Liquid Crystals, Vol. 19, No. 4, 469-480 (1995), discusses the optical properties of stretched films consisting of liquid crystal droplets disposed within a polymer matrix. He reports that the elongation of the droplets into an ellipsoidal shape, with their long axes parallel to the stretch direction, imparts an oriented birefringence (refractive index difference among the dimensional axes of the droplet) to the droplets, resulting in a relative refractive index mismatch between the dispersed and continuous phases along certain film axes, and a relative index match along the other film axes. Such liquid crystal droplets are not small as compared to visible wavelengths in the film, and thus the optical properties of such films have a substantial diffuse component to their reflective and transmissive properties. Aphonin suggests the use of these materials as a polarizing diffuser for backlit twisted nematic LCDs. However, optical films employing liquid crystals as the disperse phase are substantially limited in the degree of refractive index mismatch between the matrix phase and the dispersed phase.
Furthermore, the birefringence of the liquid crystal component of such films is typically sensitive to temperature. U.S. Pat. No. 5,268,225 (Isayev) discloses a composite laminate made from thermotropic liquid crystal polymer blends. The blend consists of two liquid crystal polymers which are immiscible with each other. The blends may be cast into a film consisting of a dispersed inclusion phase and a continuous phase. When the film is stretched, the dispersed phase forms a series of fibers whose axes are aligned in the direction of stretch. While the film is described as having improved mechanical properties, no mention is made of the optical properties of the film. However, due to their liquid crystal nature, films of this type would suffer from the infirmities of other liquid crystal materials discussed above.
Still other films have been made to exhibit desirable optical properties through the application of electric or magnetic fields. For example, U.S. Pat. No. 5,008,807 (Waters et al.) describes a liquid crystal device which consists of a layer of fibers permeated with liquid crystal material and disposed between two electrodes. A voltage across the electrodes produces an electric field which changes the birefringent properties of the liquid crystal material, resulting in various degrees of mismatch between the refractive indices of the fibers and the liquid crystal. However, the requirement of an electric or magnetic field is inconvenient and undesirable in many applications, particularly those where existing fields might produce interference.
Other optical films have been made by incorporating a dispersion of inclusions of a first polymer into a second polymer, and then stretching the resulting composite in one or two directions. U.S. Pat. No. 4,871,784 (Otonari et al.) is exemplary of this technology. The polymers are selected such that there is low adhesion between the dispersed phase and the surrounding matrix polymer, so that an elliptical void is formed around each inclusion when the film is stretched. Such voids have dimensions of the order of visible wavelengths. The refractive index mismatch between the void and the polymer in these “microvoided” films is typically quite large (about 0.5), causing substantial diffuse reflection. However, the optical properties of microvoided materials are difficult to control because of variations of the geometry of the interfaces, and it is not possible to produce a film axis for which refractive indices are relatively matched, as would be useful for polarization-sensitive optical properties. Furthermore, the voids in such material can be easily collapsed through exposure to heat and pressure.
A polarization sensitive scattering element (PSSE) has been described by in U.S. Pat. Nos. 5,751,388, 5,999,239, and 6,310,671 (Larson). Here, the PSSE is a microstructural composite of material domains having differing birefringence and wherein the PSSE transmits the majority of the light polarized along one optical axis while randomly backscattering the majority of the light polarized along a second optical axis.
Optical films have also been made wherein a dispersed phase is deterministically arranged in an ordered pattern within a continuous matrix. U.S. Pat. No. 5,217,794 (Schrenk) is exemplary of this technology. There, a lamellar polymeric film is disclosed which is made of polymeric inclusions which are large compared with wavelength on two axes, disposed within a continuous matrix of another polymeric material. The refractive index of the dispersed phase differs significantly from that of the continuous phase along one or more of the laminate's axes, and is relatively well matched along another. Because of the ordering of the dispersed phase, films of this type exhibit strong iridescence (i.e., interference-based angle dependent coloring) for instances in which they are substantially reflective. As a result, such films have seen limited use for optical applications where optical diffusion is desirable.
There thus remains a need for an improved diffusely-reflecting polarizer comprising a film having a continuous phase and a disperse phase that avoids the limitations of the prior art. The improved reflecting polarizer should have a continuous phase (the major phase) that is a relatively inexpensive material and that is amorphous, rather than crystalline or semicrystalline, to minimize haze, so the refractive index mismatch between the two phases along the material's three dimensional axes can be conveniently and permanently manipulated to achieve desirable degrees of diffuse and specular reflection and transmission. The film is also desirably stable with respect to stress, strain, temperature differences, moisture, and electric and magnetic fields, and wherein the film has an insignificant level of iridescence. These and other needs are met by the present invention, as hereinafter disclosed.